1. Field of the Invention
The present invention relates to a printing apparatus and a conveyance control method. Particularly, the present invention relates to a printing apparatus and a conveyance control method which perform accurate conveyance control even when, e.g., the leading edge or trailing edge of a printing medium enters between or passes through conveyance rollers.
2. Description of the Related Art
Recent printing apparatuses such as printers use not only plain paper but also printing media such as photo special paper to print photo images in many occasions. In particular, an inkjet printer which uses smaller ink droplets for printing can obtain an image quality equal to or higher than a film photo.
Accordingly, conveyance of printing media is also required to be accurate. Conveyance rollers use precision rollers with, e.g., a grindstone coating on a metal shaft. A DC motor used to drive the conveyance rollers is controlled by a cord wheel and an encoder sensor provided coaxially, thereby simultaneously ensuring high accuracy and high-speed conveyance.
Only one pair of conveyance rollers does not suffice for accurate printing of an image up to the trailing edge of a printing medium. To implement, e.g., marginless print, some proposed arrangements have another pair of conveyance rollers downstream in the printing medium conveyance direction. In such an arrangement, however, when the tail end of a printing medium passes through a conveyance roller pair upstream in the conveyance direction, the conveyance amount may change, resulting in density unevenness in the image. To ensure conveyance accuracy up to the tail end of a printing medium, the nozzles of the printhead to be used are restricted, or printing medium conveyance is controlled in addition to the use nozzle restriction during printing on the tail end part of a printing medium, thereby maintain the printing quality (see Japanese Patent Publication Laid-Open No. 2002-254736). The mechanical accuracy of the conveyance roller pair downstream in the conveyance direction is also raised to ensure the conveyance accuracy.
In recent years, the requirements for a higher printed image quality and a higher printing speed keeps growing. To meet these requirement and attain higher image quality, ink droplets to be used in printing are becoming smaller. This also indicates that it is necessary to more accurately convey a printing medium.
For this purpose, preferably, a cord wheel is axially provided not only on the upstream precision conveyance roller but also on the downstream conveyance roller, and dedicated encoder sensors are used such that a plurality of encoder sensors control the upstream and downstream roller positions, respectively.
However, employment of such a double encoder configuration makes it difficult to accurately control conveyance when the tail end of the printing medium has passed through the upstream conveyance roller, or the front end of the printing medium has reached the downstream conveyance roller. Accurate conveyance control is also impossible even when the tail end of the printing medium is just going to pass through the upstream conveyance roller. For example, if the tail end of a printing medium is located closest to the nip of the upstream conveyance roller pair, the printing medium may be fed by an amount corresponding to the backlash of a series of drive gears to the downstream discharge roller. In this case, the print position may shift by an amount equal to the conveyance amount, resulting in poor quality of a printed image.